Known radio frequency and microwave antennas used in handheld, laptop computer, and vehicular communication systems, for example, have a ground plane either by necessity, such as the ground plane of the printed circuit board in a cellular telephone or the metal surface of the vehicle, or intentionally to direct most of the radiation to a particular hemisphere of the antenna. The ground plane, however, can degrade the performance of the antenna unless the antenna is placed a quarter wavelength away from the ground plane, which results in constructive interference between the radiated and reflected fields. In addition, propagating surface waves lead to the undesirable characteristics of power loss, variations in radiation patterns due to ground plane edge effects, and, when antennas share a common ground plane, coupling that can result in unwanted radiation pattern nulls.
In order to eliminate or reduce these problems, a known high impedance surface is used to replace the ground plane, and is placed near the antenna. Unlike a typical highly conductive ground plane, the high impedance surface has a boundary condition that supports a tangential electric field. The antenna can then be placed closer to the high impedance surface since image currents are not phase reversed, which is a consequence of enforcing the boundary condition on the tangential electric field. The high impedance surface also reduces power consumption by improving the impedance match, reduces interference or coupling with other antennas on the platform, and can increase the usable frequency range of an antenna.
There are problems, however, associated with such prior art high impedance surfaces. Known high impedance surfaces, for example, often support only a limited resonant frequency range and typically are not compatible with frequencies less than ten GHz. This, of course, presents problems for platforms that rely upon lower frequency bands, such as many commercial and military wireless applications in the commercial cellular (about 800-2500 MHz), ultra high frequency (UHF) (about 300-1000 MHz), and very high frequency (VHF) (less than 300 MHz) bands. Many existing high impedance surfaces are also relatively thick, which is undesirable for many applications for which thickness and weight must be minimized.